1. Field of the Invention
The present invention relates to a hydraulic transmission system that includes right and left hydrostatic transaxles supporting respective right and left axles so as to be applicable to a zero-turn vehicle, such as a lawn mower.
2. Related Art
US 2009/0301076 A1 teaches a lawn mower, serving as a typical zero-turn working vehicle, which is equipped with a hydraulic transmission system including right and left hydrostatic transaxles (integral hydrostatic transaxles, i.e., “IHT”s) supporting respective right and left axles, so that the right and left transaxles are available to differentially control the right and left axles, thereby enabling zero-turn of the vehicle. Each of the transaxles includes a casing that incorporates a hydrostatic stepless transmission (HST) and supports the corresponding axle. The HST includes a hydraulic pump, a hydraulic motor for driving the corresponding axle, a hydraulic circuit (HST circuit) fluidly connecting the hydraulic pump to the hydraulic motor, and a charge system for charging fluid to the HST circuit. The charge system includes a charge pump and charge check valves.
In the above-mentioned conventional hydraulic transmission system for a zero-turn vehicle, the casing of each transaxle has a pair of ports. One port is used for extracting fluid delivered from the charge pump to a hydraulic implement, such as a hydraulic cylinder for lifting a mower unit, disposed outside of the casing, and the other port is used for introducing fluid from the hydraulic implement to the charge check valves, thereby charging fluid to the HST circuit via one of the charge check valves.
The main reason why the casing of each transaxle has the pair of ports for extracting fluid to the outside of the casing and for returning fluid to the inside of the casing is to effectively cool the fluid while the fluid flows in pipes that are extended outward from the casing to connect the ports to the hydraulic implement. However, the pipes must be expensive high pressure pipes, such as metal pipes.
US 2011/0162355 A1 (hereinafter referred to as the '355 reference) also teaches a hydraulic transmission system for a zero-turn vehicle, including right and left transaxles (IHTs). In this system, a pipe is interposed between ports on the upper portions of casings of the respective right and left transaxles, and an external reservoir tank is provided on an intermediate portion of the pipe. In other words, the external reservoir tank divides the pipe into portions connected to the respective ports, while another pipe is interposed between low portions, e.g., oil pans, of the casings of the respective right and left transaxles. The pipe interposed between the ports on the upper portions of the casings of the right and left transaxles can be made of a cheap low pressure pipe, e.g., a rubber hose, because it has to bear against only a low pressure of fluid overflowing from the casing of either the right or left transaxle or only a low pressure of fluid flowing from the external reservoir tank to the casing of either the right or left transaxle caused by hydraulic depression of a fluid sump in this casing. On the other hand, the pipe interposed between the ports on the lower portions of the casings of the right and left transaxles must be an expensive high pressure pipe because it must bear against forcible flow of hydraulic fluid from one casing to the other casing. However, due to the above-mentioned low pressure pipe for allowing fluid to overflow from one transaxle to the other transaxle, the high pressure pipe can be reduced in number so as to reduce costs.
The '355 reference discloses some embodiments of the high pressure pipe between lower portions of the right and left transaxles. In one embodiment (FIG. 6), a charge pressure regulation valve is disposed in a casing of one transaxle (referred to as a first transaxle) so as to be connected to a charge passage interposed between a charge pump and charge check valves, thereby supplying the charge check valves with fluid having a pressure regulated by the charge pressure regulation valve. In this regard, in the casing of the first transaxle, a relief passage branches from the charge passage and extends to the port connected to the high pressure pipe, and the charge pressure regulation valve is disposed on the relief passage so as to release fluid to the port connected to the high pressure pipe and so as to drain the released fluid to a fluid sump of the other transaxle (referred to as a second transaxle). Therefore, part of the fluid released from the charge pressure regulation valve in the first transaxle is drained via the high pressure pipe to the fluid sump in the second transaxle, so as to increase the volume of the fluid sump in the second transaxle, whereby fluid of the fluid sump in the second transaxle overflows to the external reservoir tank via one portion of the low pressure pipe, and is introduced from the external reservoir tank to the fluid sump of the first transaxle via the other portion of the low pressure pipe by hydraulic depression of the fluid sump of the first transaxle, thereby circulating hydraulic fluid between the first and second transaxles so as to radiate heat from the hydraulic fluid while flowing in the high and low pressure pipes.
Further, in the casing of the first transaxle, a drain passage branches from the relief passage at a downstream side of the charge pressure regulation valve and extends to a fluid sump of the first transaxle so as to drain surplus fluid from the relief passage to the fluid sump of the first transaxle. In this embodiment, a hydraulic actuator is disposed on an intermediate portion of the high pressure pipe between the ports on the lower portions of the casings of the first and second transaxles, so that the flow of fluid in the high pressure pipe from the port of the first transaxle to the port of the second transaxle must pass through the hydraulic actuator. In particular, the above-mentioned drain passage in the casing of the first transaxle is intended to drain fluid having backpressure from the hydraulic actuator when the moved hydraulic actuator reaches a limit position of its movement. However, it seems that, regardless of whether or not the hydraulic actuator reaches the limit position, fluid released from the charge pressure regulation valve in the first transaxle is liable to flow into the drain passage in the first transaxle rather than to flow into the high pressure pipe having the hydraulic actuator resisting the flow of fluid in the high pressure pipe, so that the flow of fluid in the high pressure pipe may be insufficient to smoothly operate the hydraulic actuator and to cool the entire hydraulic transmission system as expected. No other embodiment in the '355 reference teaches how to solve this problem.
Further, in some other embodiments (FIGS. 3 and 4) of the '355 reference, the ports with the high pressure pipe therebetween are fluidly connected to fluid sumps in the respective casings. The high pressure pipe is provided on an intermediate portion thereof with no hydraulic actuator, however, with an additional charge pump to make sure fluid flows from one transaxle to the other transaxle, thereby increasing costs.